1. Field of the Invention
The present invention relates to a method of fabricating thermoelectric materials using core-shell structured nano-particles, and thermoelectric materials fabricated by the same.
2. Description of the Related Art
Thermoelectric materials are energy conversion materials that create electric potential when there is a temperature difference between both ends or create temperature difference when there is electric potential. In early 19 century, thermoelectric effect was specified as Seebeck effect, Peltier effect and Thomson effect. Since late 1930's, the thermoelectric materials have been used for a special independent power supply using thermoelectric generator in the field of aerospace and military, mainly in U.S. and Europe. Nowadays, a generator using heat and waste heat of a car or incinerator is developed. Also, the thermoelectric materials are widely used for a refrigerator/heating cabinet, a small sized air-condition system and a small sized cooler related to a computer to control the temperature precisely and a cooler/heat exchanger of hot & cold dispenser, and so on.
The generating and cooling capacity of thermoelectric materials are represented as a figure of merit given by ZT=(σα2/κ)T, wherein α is Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature. High energy conversion efficiency of thermoelectric materials indicates high figure of merit (ZT). In order to ensure high ZT, electrical conductivity and Seebeck coefficient need to be improved, while thermal conductivity has to be decreased. That is, the characteristics of thermoelectric elements need to be controlled independently to have high electrical conductivity and low thermal conductivity.
Among the variables to determine a figure of merit (ZT), Seebeck coefficient and electrical conductivity are related to the characteristics of transferring charges, and thermal conductivity depends on transferring of phonon. Considering this, the characteristics of thermoelectric materials need to be controlled by micro-structure control. That is, the scattering of charge is minimized within thermoelectric materials to maintain or improve electrical conductivity and the scattering of phonon is maximized to induce the decrease of thermal conductivity, and thus, the figure of merit can be improved.
Meanwhile, the research introducing dispersed phase to thermoelectric matrix phase has recently been studied to fabricate the thermoelectric materials having higher figure of merit (ZT). That is, since a new interface of dispersed phase/thermoelectric phase which is formed by grain boundary and dispersed phase of thermoelectric materials actively induces scattering of phonon by introducing nanometer-sized dispersed phase to thermoelectric matrix materials, thermal conductivity thereof efficiently becomes lower and thus, the figure of merit (ZT) can be improved. Since the phonon wavelength is 1-2 nm and the wavelength of charge is 10-nm, using nano-particles sized below 10 nm causes electrical conductivity to be maintained and thermal conductivity to be decreased efficiently. Also, if the size of dispersed phase (nano-particles) is kept below 10 nm, which does not have an affect on the movement of carrier, the problem involved in lowering conductivity as a result of introducing the dispersed phase can be solved. Accordingly, the research of fabricating inner type thermoelectric materials having nano-dispersed phase in the inner grain of thermoelectric matrix materials and thus improving a figure of merit (ZT) has continuously been studied.
L-D. Zhao et al (Thermoelectric and mechanical properties of nano-SiC-dispersed Bi2Te3 fabricated by mechanical alloying and spark plasma sintering, Journal of Alloys and Compounds 455 (2008) 259-264) mainly teaches a method of mechanical milling thermoelectric powder, such as Bi2Te3 and nano-dispersed phase to disperse nano-dispersed phase onto thermoelectric matrix phase. However, the thermoelectric elements fabricated by mechanical milling have some problems including agglomeration of nano-dispersed phase and irregular dispersion of nano-phase (i.e., nano-phase is dispersed only for inter-grains of powder), and thus, there is limitation to improve a figure of merit (ZT).
If nano-phase is dispersed only for inter-grains of powder, when fabricating thermoelectric materials by sintering the powder, scattering effect of phonon is mainly performed in grain boundary, while the nano-dispersed phase concentrated on the grain boundary forms a new interface resulted as an effect of dispersed phase, so that it is difficult to induce scattering of phonon.
Korean Patent No. 10-0795194 (granted on Jan. 9, 2008) teaches “method for fabricating thermoelectric materials by mechanical milling-mixing and thermoelectric materials fabricated thereby” and provides a method of fabricating thermoelectric materials in which thermal/electrical characteristics are controlled by mechanical milling for differencing the size of thermoelectric materials, mixing the milled materials, shaping and sintering the materials, and thermoelectric materials fabricated by the same. Although the granted patent contributes to crystallizing and nano-structure of single elements, the patent cannot solve agglomeration of nano-sized dispersed phase by mechanical mixing and since the dispersed phase exists on the surface of Bi—Te metal powder rather than inside of Bi—Te metal powder, the patent cannot be applied to a method of fabricating the thermoelectric materials using nano-dispersed phase as sought after by the present invention.
In addition, Korean Patent Publication No. 10-2011-0018102 (Feb. 23, 2011) describes thermoelectric matrix along with ceramic nano-particles; and composite thermoelectric materials in which the thermoelectric material-matrix and ceramic nano-particles are combined by bi-polar dispersant.
Further, Korean Patent Publication No. 10-2011-0052225 (May 18, 2011) describes nano-composite thermoelectric materials including thermoelectric material-matrix; and metal nano-particles which have higher electrical conductivity than the above mentioned thermoelectric materials and is dispersed in combination therewith.
As explained above, researches to improve figures of merit (ZT) by dispersing nano-dispersed phase within thermoelectric materials have been studied.
In this regard, while researching a method of improving a figure of merit of thermoelectric materials, the inventors of the present invention prepared core-shell structured nano-particles and developed a method of forming thermoelectric materials on the surface of the prepared core-shell structured nano-particles, and thus, completed the present invention.